1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device.
2. Description of the Related Art
There is a strong demand for high density mounting of electronic parts, and the bare chip assembly system is being watched with keen interest. The connecting structure in the bare chip assembly has been changed from face-up mounting using a wire bonding method to face-down mounting using the flip chip joint and solder bumps. In the joint using the solder bumps, the solder bumps are formed on electrodes on the surface of a semiconductor element. Concerning the method of forming the solder bumps, a method using electrolytic plating is known (For example, refer to Japanese Unexamined Patent Publication (Kokai) No. 6-133382 (pages 3 to 4, FIG. 1)).
As shown in FIG. 5A in the attached drawings, in the conventional method of forming solder bumps, a resist (insulating film) 56 is provided on the surface of a semiconductor substrate 50, and openings are provided at the positions of the electrodes in the resist on the semiconductor substrate. Then, the metal 58, which becomes solder bumps, is supplied into the openings by means of electrolytic plating. This metal 58 is formed to a height so that the metal 58 protrudes from the surface of the resist.
According to this method, the larger the thickness of the resist is, the higher the bumps it is possible to form. In the case of forming the solder bumps at narrow pitches, the area of the solder bump becomes smaller, and therefore, it is preferable that the height of the solder bumps is increased so as to ensure a required quantity of solder. When the solder bumps are formed by means of electrolytic plating, it is possible to form solder bumps having a large height at narrow pitches at a relatively low manufacturing cost.
The resist can be provided by coating liquid photo-resist by means of spin coat, of alternatively, it is possible to use dry film resist of uniform thickness. As dry film resist having a large thickness can be easily procured, it is appropriate for forming tall bumps.
In the case where the thick resist is used, a problem is caused in which it is difficult to separate the resist from a semiconductor substrate. In the case where metal, which becomes solder bumps, is formed to such a height that it protrudes from the resist surface, a portion of the plated metal covers the resist surface, and the profile of the plated metal becomes a mushroom-shape, as shown in FIG. 5B. Therefore, it is not easy for a release agent to get between the resist and the plated metal and between the resist and the semiconductor substrate. In the case of using the dry film resist, the dry film resist is separated by causing an alkali solution to penetrate into the dry film resist so that the dry film resist can be swollen. If it is not easy for the release agent to penetrate between the resist and the plated metal and between the resist and the semiconductor substrate, the resist cannot be perfectly separated, and the remnants 60 of the resist remain in the peripheries of the solder bumps and on the surface of the semiconductor substrate in some cases. As the resist remnants 60 absorb moisture, ion migration occurs, which could be a cause of short circuits between the bumps. Therefore, in order not to leave the resist remnants, it is necessary to reduce the height of the solder bumps to be smaller than the thickness of the resist. For the above reasons, in the case where the solder bumps are formed at narrow pitches, it is difficult to ensure a quantity of solder required. When the quantity of solder is low, joining will become defective or the capacity of alleviating stress by the solder bumps is deteriorated, that is, the reliability of joining is lowered.